what’s a good dbm for a phone call (GSM or UMTS)

Basically, for GSM:
-105 to -100 = Bad/drop call
-99 to -90 = Getting bad/signal may break up
-89 to -80 = OK/shouldn’t have problems, but maybe
-79 to -65 = Good
Over -65 = Excellent

More here:

http://www.howardforums.com/showthread.php/1523050-dBm-signal-strength-on-GSM-acceptable-numbers

And how do you find the dbm of your phone signal?

On Android, *#*#info#*#*

i.e. *#*#4636#*#*

It will automatically go into a very detailed set of information screens. No need to tap the Dial button.

https://www.quora.com/Which-brand-of-mobile-phone-do-real-techies-use-and-why/answer/Leonid-S.-Knyshov?srid=3soS