Hi starrysky,
It would be
majorly more concerning if you were real casual about the phone counseling
Your feelings, right now, are saying that you're taking it seriously and that you care about what you'll be doing and the people you'll be talking to, and that can help you make a real difference to people when they need it most, that matters
so much.
But let's try to turn those feelings into a tool to help you in moving forward to where you want to be, hey??

Maybe write down some of your concerns and any "worst case scenarios" that are going through your mind.........then they are out there, something real tangible to focus on,
and then...........I'd say discuss them with the supervisor/organiser/contact you have there.........my guess is that a lot of those fears will be natural worries for anyone who takes the position seriously, but it may turn out to be really reassuring to hear the advice/answers/support from your contact there who should be real experienced in allaying a lot of those fears in a lot of other newly appointed volunteers.
And
no-one could expect you to have all the tools to handle any situations right now, to feel confident about gradually just "getting on with it", but
that's OK,
that's fine 
Training will give you a lot of those tools and equip you a lot more in handling situations.
And there
will be support systems in place, even when you've started for
however long you're there for.
So.........right now, you are
not in "at the deep end", and you're not going to be.........you are going to be
so prepared in asking about any concerning situations and getting advice on those, you're going to be getting the training, tools and support to equip you in deciding if this is for you, and you do have a
massive chunk of what it takes to make it work for you if you want that..........you do take it seriously, and you
do care
And one step at a time, hey??

It's just great that you're giving it a shot, and
massive kudos to you for that.
If it works out, fantastic, but equally if it doesn't maybe the process and some of the training could still really benefit you in interacting with some people in your life right now or in the future............
It's
all positive starrysky
As for the physical therapy........maybe you could talk to your therapist about how this may be made a little easier for you in
whatever ways..........but keep on, and on, and on reminding yourself of the purposes of it/the goals you're reaching for, hey??

AND the achievements you've made, however small/timely they may seem, they are
still achievements, they
still matter
Alison