I'm not sure what country you guys are in.
But I've found over the years a few handy things to do and not do.
Have an unlisted telephone number. Or change your current one to an unlisted one. This invalidates peoples existing databases. I'm not on any barring list.
Do not hand out your telephone number to all and sundry on business reply cards, website signups etc. Remember fake phone numbers are more difficult to check from logins on web sites, from the web site PoV, than emails. Mobile numbers seem to be more valuable than land line #s so be especially careful who you release yours to. Dealing with normal scumcompanies I'll deny I even have a mobile to call.
Remember these people are much more desperate to contact you than you are to hear from them.
Pretty much the only spam calls I get are from companies that I already deal with who think I want to extend my "relationship" (IE buy more s**t) with them than I already have.
I don't enter competitions or prize draws. So any BS about "This is an important message" -> hangup.
Ask yourself "If this is an important message from a company you deal with can they contact me another way (IE email, which is fast and does not need a user present to receive it) and if so why have they not done so?"
I go months without getting one of automated spammers and cannot remember the last one I had.
As far as I'm concerned most companies like most governments are
No need to ask. No "need" to know
And yes the biggest source of spam on my mobile is the network operator
Fail because this is BT. Home of Phorn, ignoring guidelines on premium rate phone numbers etc. WTF should you pay for a solution to a problem they cause.