I don't know if this would work or not, but before you leave your local calling area, couldn't you just go into Options > Call Forwarding in the Phone module and have your messages forwarded to a voicemail service which sends you an email containing the voicemail as a wav or mp3 file? As I understand T-Mobile's billing, you get 500 minutes each month for calls which are forwarded using the Forward Unanswered Calls option. I don't think you would be charged for the voicemail, because you never have to use the phone service to check it.
From Tmobile's website:
Using Voice Mail While Roaming
NOTE: If you check voice mail from your phone while roaming, you will be charged for roaming and long distance, as follows:
* Domestic GSM Roaming = $0.49/minute airtime + $0.20/minute for domestic long distance International GSM Roaming
* International GSM Roaming = Airtime rates will vary depending on the country in which you are roaming. For more information on international roaming and pricing, go to International Services.
There are multiple ways to call your voice mail while roaming. Option 1 is the primary method and should always be used first. Options 2 and Option 3 should only be used when Option 1 is not successful. Long distance fees will apply for the duration of the call.
There are a number of free options for wav or mp3 voicemail services. Try k7.net (wav) or freedigits.com (mp3). I have accounts at both. I prefer k7.net because the email message you receive includes the phone number of the caller in the subject line (if that number is available to caller id services). However, Freedigits is faster on sending emails notifying you of your voicemail. K7.net can be delayed for hours occasionally. As I've mentioned in other posts, you can use your Pearl browser to listen to your voicemail message.
I know that the next time I'm going to be roaming internationally, I will change my voicemail message at k7.net or freedigits to tell users to leave their email address when they leave their message. That way, unless I actually need to talk with them, I can email a reply. I might even use the voicenote feature.
Using the $19.95 international blackberry option and a system like I've described might work well. Of course, you will still get dinged when you use your blackberry as a phone.