It's useful to distinguish between literal translation and functionally equivalent translation even when you're dealing with rather literal language. You note that the English phrase is a cliché as well as an euphemism, which makes it doubly idiomatic (non-literal), suggesting double caution. In this case we're in some luck, as there exists at least partial equivalence (see below). But first I'll discuss the literal translation:
- tempus with or without an adjective (bonum, malum) isn't used to mean "an experience". Instead adverbial predicative expressions with bene, lepidē, pulchrē are used (see this answer). One of the examples I provided there is highly relevant:
Ubi tū lepidē volĕs esse tibī, “mea rosa—” mihi dīcitō, // “—dato quī bene sīt” : ego ubī bene sīt tibi locŭm lepidum // dabō. ('When you want to have a lovely time, say to me, “my rose, give me some fun”; I’ll give you a lovely place where you can have some fun.' Plautus, The Two Bacchises 82-83, tr. Wolfgang de Melo)
- prō indeed isn't used to mean "in order to", but rather "in exchange for; instead of" and hence "as a, for a, as equivalent to". What you need is a verbal clause introduced with ut: ut bene sit tibī (or quī for ut as above, both more colloquial/idiomatic and limited).
- but here the English sentence actually expresses a condition ("if you want to have a good time"), which normally will be expressed with ubī/sī.
So as a literal equivalent of "for a good time" we get:
- ubī/sī lepidē/pulchrē/bene/bellē volēs esse (tibī)
Now that we have a largely equivalent euphemistic expression for giving/receiving sexual entertainment, the question to ask is whether such expressions were used as clichéd advertisements. My familiarity with graffiti is limited, but none of the ones I've seen that can be (and commonly are) interpreted as ads for sexual services use such euphemisms. The Roman society is known for having been fairly direct, and their language reflects this; the sex ads of Pompeii and Herculaneum are all of the literal variety (a useful sampling):
- A(SSIBVS) IIIII NICE FELLAT ('Nike will suck you off for 5 pounds' CIL IV 2278)
- MENANDER | BELLIS MORIBVS | AERIS ASS(IBVS) II ('Menander, a real gentleman, (will do you for) for 2 pounds' CIL IV 4024)
- MENTVLA V HS ('Dick, 5 sesterces' CIL IV 8483, which might be a parody due to the organ-as-name and the exorbitant price; parody presupposes a cliché)
- MARITIMVS CVNNV LIGET IIII VIRGINES AMMITTIT ('Maritimus licks pussy for 4 pounds. Virgins are fine' CIL IV 8940)
There's a good deal of cultural component involved here, but I would argue that this is also the case with the T-V distinction, the use of honorifics as well as politeness markers, e.g. in questions. Thus introducing euphemisms where none were used by the Romans would un-Latin as well as un-Roman.
Finally there's the issue of which verb to use for "call, dial by phone" - I'm not sure there's a universally accepted option, and would rather see this discussed in a separate question. The graffiti of the phoneless world seem to use quaerere for this (quī volet—quaerat, sī volēs—quaerēs).