There really is no reliable way to get your number to him without sending it in writing.
Facilities do not allow staff to pass along personal messages like phone numbers. If you call, most will politely refuse. Occasionally someone might help, but you cannot count on that.
Your best and fastest options:
1. Send a letter (most reliable)
2. Use a mutual contact
If he has:
They can pass your number along.
3. Send a quick postcard or short note
This is often faster than waiting or hoping staff will help. It is a small cost and the only method that consistently works.
Why this is the reality:
About adding money first:
You are right to hesitate. It is smart to wait until he has your number and can actually call.
Bottom line
Mail is the only dependable way to get your number to him. It is simple, cheap, and it works.