You're doing a good job, with the texts and calling, if you're not sure, holding back and being a bit calculated about what move to take next. Rather than giving in to some insecurity and rushing in and sending a bunch of needy texts and missed phone calls.
Yeah you're right, the next move should have been something very casual. I would have called when I got the text because there's a good chance she'll pick up, and you can do 100 times better a job when you're voice is on the line, rather than text.
At least she believes you only called cause your friend bailed. There's nothing to remedy.
Next step is to call, and if it rings out, send a text letting her know you just tried to catch her (very important to highlight that the call rang out and this is an immediate follow up text, not some sort of badgering, call, then text... which may be followed by more calls or texts), and say that it's a shame her friends had to leave so early because you were enjoying the chat and want to continue it sometime. Boldly offer to catch up over coffee after work, and the subtext is all set up fine, so that she will respond with another time if she can't make that.
Eg. "Hey just tried to get a hold of you - I was just thinking, it's a shame your friends bailed just when I was starting to enjoy our little chat! Want to continue sometime, maybe over coffee after work on Monday?" Of course if you don't work near each other, you have to offer a good time and place. If you're not sure where she works, you'd end it with "...maybe over coffee after work on Monday? Where do you work?"
And if you can, you should always personalise it, say "I hope X went well" etc. something from your last chat, as long as it doesn't seem too forced in.
This all has to be your own words though. I don't think I've even sent that message verbatim, that was just something generic I thought up now - I tailor it as much as possible, keep it short, and make sure I cover the points. In truth I may re-write it to say the same thing in fewer words
But if you over-do that, it can sound unnatural. It still has to sound like you just typed it out real quick - without being overly casual. Hahah. I used to pingpong back and forth like that, but over time it's become heaps easier to just write it short and snappy in the first go or two. It's about getting better at saying what you mean.Anyway - once those points are covered, the subtext is this is really casual, you want to see where this goes, coffee is just an off-hand idea to get you two talking again, nothing serious, and even if she's not free she'd say "Coffee sounds great - I can't do Monday though, how about Tuesday?"