1.  Yes, you should always ask first.

2.  Only if the provider is OK with phone numbers.  Most prefer email.

3.  Yes, email is always better than phone if you are not sure.

1) if she asked for references from you to see you and/or if she states she is reference friendly on website or ad, you should be safe to assume she is reference friendly but its better safe than sorry ... I suggest you ask when you meet her in person if she minds ... I often to the initiative and let my clients know that they can use me as a reference especially if they only had a couple of them. Also I've had some clients email me first before they give out my info and that's nice if we haven't discussed it before or if its been a long time since I've seen them.  However, too many emails can take up too much time so if she says yes once then you should be good for future as long as its not more than 6 months since the last reference or meeting.

2) if her number is on her website or ads then you should be safe but best to give as much contact info as you can such as number, email address, ter #, P411 link .... As long as these are easily located on her website and ad.

3) if email is what she states that she prefers then yes but also as I stated in #2 as much info as you can especially ter and p411 links ... I research those who ask for references before I give any info on my clients unless I know the provider .... I actually prefer reference requests via ter or p411 so I can verify them in 2 seconds.

Long story short... Yes make sure it's okay however you do that, check with her website or her personally to find out the best method of contact, and always include as much information as you know she would be comfortable with to the new provider so that she has more than one way to get ahold of your reference.

because many will not answer a message from another lady...  for all they know, it could be your SO or her PI trying to catch you or get information.