Short answer: Write down and plan what you want to say beforehand!
I used to have similar issues with speaking on a voice-only medium such as Skype or conference calls.
What worked for me was to have a clear idea beforehand as to what I wanted to say, and write it down. If you find a question gets raised during the meeting, then write it down.
So why write it down? Because it gives you a chance to look at the point, then formulate what you want to say or ask. It's normally being unsure of what you're going to say that causes the hesitation to speak. Writing it down clarifies it in your mind so you can speak with confidence.
Once you have planned what you want to say, then when there is either a break in the discussion or a relevant point to interject, look at your notes, remember what you planned to say and speak clearly and succinctly.
I've been doing this for the better part of 20 years and I find that am now effective in my communication in Skype meetings.